The present invention is generally related to the field of Rapid Thermal Processing and, more particularly, to techniques and apparatus for enhancing uniformity in Rapid Thermal Processing.
The prior art has developed a number of approaches in the use of Rapid Thermal Processing (RTP) for purposes of producing state-of-the-art devices. Such devices include, for example, semiconductor chips, solar cells and nanoparticle materials. RTP has been found to be particularly useful following ion implantation processes applied to a substrate or workpiece, in which the implanted dopant atoms are left in interstitial sites where they are electrically inactive. RTP is applied with the intention of repairing the damage to the crystal lattice structure and to move dopants to lattice sites to electrically activate these dopants.
One form of RTP essentially uses isothermal heating in which radiant energy is applied for a time duration and at a cooperating intensity so as to cause the temperature of the workpiece to rise at least approximately uniformly throughout the bulk of the workpiece thickness. Thus, this form of isothermal RTP may be referred to as “isothermal RTP” and is generally characterized by treatment time durations on the order of seconds.
As device sizes and junction depths have progressively decreased with successive generations, difficulties have come to light with respect to the use of isothermal RTP. In particular, at the high temperatures that cause the desired dopant activation, it has been found that diffusion mechanisms come into play which cause the dopant impurities and other species to diffuse from their intended positions within the overall device structure. Such diffusion can result in impaired functionality of the device, in view of reduced feature sizes.
Concern with respect to undesired diffusion effects has motivated the development of what may be referred to as millisecond or flash RTP. This more recent approach to RTP is characterized by heating the workpiece in a way that deliberately produces a temperature gradient through the thickness of the wafer or workpiece. One highly advantageous approach is described in U.S. Pat. No. 6,594,446 entitled HEAT TREATING METHODS AND SYSTEMS, which is incorporated herein by reference in its entirety. The time duration, for purposes of millisecond RTP, is from 0.1 ms to 20 ms. The premise of millisecond RTP resides in flash heating the device side of the workpiece briefly so that the bulk of the workpiece remains cooler. In this way, the bulk of the workpieces acts as a heat sink, following flash heating of the device side of the workpiece. Such an implementation is effective when the time period of the flash heating is considerably faster than the thermal conduction time of the workpiece.
Thus, millisecond RTP reduces dopant diffusion by limiting both the time at which temperature is sufficiently high to enable diffusion, in conjunction with limiting the volume of workpiece that is heated to such high temperature. Of course, pulsed RTP may be used in what may be considered as a hybrid form with isothermal RTP, for example, by heating the workpiece to an intermediate temperature and then applying pulsed energy as is taught in U.S. Pat. No. 6,849,831 which is incorporated herein by reference.
In addition to the aforedescribed difference with respect to diffusion effects, each form of RTP introduces other unique problems and opportunities with respect to its application. For example, with respect to isothermal RTP, heating uniformity is a concern, particularly for the reason that peripheral edges of the workpiece tend to lose energy more rapidly than the central portion of the workpiece. The edge region, therefore, tends to remain cooler than the center of the workpiece. As an example with respect to millisecond RTP, surface heating is intended to be essentially instantaneous. Therefore, pulse parameters must be carefully determined in advance to produce an intended result and there is generally no opportunity to influence the process result, once the pulse has been initiated. In contrast, it is recognized that edge cooling, in millisecond RTP, is generally of limited concern, since heating rates are generally extremely high in comparison with the rates of radiative losses or conduction and convection heat losses from the workpiece to the ambient gas in the treatment chamber.
With the foregoing in mind, U.S. Pat. No. 4,981,815 (hereinafter the '815 patent) provides one example of isothermal RTP which attempts to resolve the problem of edge cooling. In one approach, that is illustrated by FIG. 7 of the '815 patent, a heating arrangement is used that employs one heat source in a confronting relationship with a major surface of the workpiece and another, separate heat source in a confronting relationship with the peripheral edge of the workpiece. It is submitted that this approach may be unduly complex in its need for an additional heat source that requires precision control. As one alternative, FIG. 6 of the '815 patent illustrates a reflector that is situated around the peripheral edge of the workpiece for returning thermal energy that is radiated from the peripheral edge of the workpiece. In this regard, it is of note that the prior art contains many examples based on reflecting thermal energy that is radiated from the workpiece back to its peripheral edge. It is considered that this approach is generally problematic for the reason that the radiated energy that is returned is simply not sufficient to compensate for the edge cooling effect caused by a combination of radiative, conductive and convective heat loss.
Another isothermal RTP approach is illustrated by FIG. 10 of the '815 patent, which uses a heating arrangement in a confronting relationship with a major surface of the workpiece, housed within a reflective box. The workpiece is movably positioned on support pins relative to a bottom wall of the reflective box such that varying the height of the workpiece, relative to the bottom reflective wall, allows a varying amount of reflected heat source energy to reach the bottom, peripheral edge region of the workpiece. FIG. 11 of the '815 patent illustrates yet another approach to isothermal RTP which is related to the approach of FIG. 10 of the patent at least to the extent that the workpiece is supported for movement on support posts. This movement is assertedly used to vary heating of the workpiece edges during the heating interval. Applicants recognize that these support posts, unfortunately, in isothermal RTP, will generally produce cold spots on the opposing surface of the workpiece. The patent, however, does not address this problem, as will be further discussed at an appropriate point below. Further, the reflective surface extends under the wafer and it is considered that this reflector would interfere with a double-sided heating implementation.
U.S. Pat. No. 4,560,420, issued to Lord, includes one embodiment that is deemed as suitable for rapid thermal annealing, illustrated in FIG. 5 of the Lord patent. This figure illustrates a raised reflective ring that is formed in the oven floor and situated directly under the peripheral edge of the workpiece. The perimeter wall of the reflective ring is made diffusely reflective while the interior, raised surface, that is surrounded by the reflective ring, receives a heat-absorptive black coating. Assertedly, the interior raised region cools the central portion of the workpiece with the intention of reducing temperature variations across the wafer. Other embodiments disclosed by Lord in FIGS. 2-4 of the patent are described as being prone to thermal cycling during heating. Hence, it is submitted that they are not well-suited for use in RTP processes, since these configurations will absorb energy from the heating arrangement and continue to reradiate thermal energy onto the workpiece edge, even after the heating arrangement is shut down. More particularly, the embodiment of FIG. 2 relies exclusively on the mechanism of returning thermal energy, that is radiated therefrom, to the workpiece edge, in conjunction with the mechanism of reradiating heat source energy. The latter mechanism is performed by absorbing energy from the heating arrangement, using at least one surface that is parallel with the major surfaces of the workpiece, and reradiating this energy from a surface that is in a confronting relationship with the peripheral edge of the workpiece. That is, there is no mechanism for producing reflection of the heat source energy onto the peripheral edge of the workpiece.
Still considering the Lord patent, it is of interest to note that the embodiments of FIGS. 3-5 are inherently limited to use in single sided workpiece heating configurations. That is, the reflector/radiator structures that are shown are opaque and located directly underneath the peripheral edge of the workpiece. Such structures would introduce problematic shadowing if any attempt were made to illuminate the bottom surface of the workpiece, as illustrated in the views of these figures.
The present invention is considered to resolve the foregoing difficulties and concerns while providing still further advantages.